The present invention relates generally to methods and devices for monitoring the gas volumes of storage tanks.
A known hydropneumatic storage tank is disclosed in the published German patent application No. 2,431,605 which includes a pressure cylinder. The inside space of the pressure cylinder is divided by a partition, preferably by a free moving piston, into two chambers. One chamber is filled with a compressible gaseous medium such as nitrogen, and the other chamber is filled to a greater or lesser extent with an hydraulic fluid such as oil at a specified amount depending on the existing working conditions. The German patent discloses a gas-pneumatic, fluid-pressure storage unit or a hydro-pneumatic storage unit having a free piston arranged in the pressure cylinder between one fluid-pressure chamber and one gas-pressure chamber.
A permanent magnet moves alongside the pressure cylinder in relation to the position of the free piston, with the pressure cylinder being of a magnetizable metal, preferably steel. One of the cylinder covers is made of a non-magnetic material, and there is imbedded in at least one blind-end bore of the cylinder cover a magnetically actuated switch, preferably a reed switch in a sealed envelope. The reed switch forms a part of an electric circuit to monitor the position of the free piston. The moving permanent magnetic is placed in the pressure chamber which is closed off by the non-magnetic cylinder cover. An engaging device is provided between the free piston and the permanent magnet which engaging device will move the permanent magnet into the switched-on position when the spacing of the free piston with the cylinder cover is less than a predetermined minimum value. Such a gas-pneumatic, fluid-pressure storage tank, however, can not be readily installed or replaced, as is typically necessary in the case of standard storage tanks. Any installation at a later time of such tanks involves considerable changes and work on the drive system of the storage tanks, especially if the storage tank includes an energy-storage system for an electrical switch gear.
Even a simplification of the above-described gas-pneumatic fluid-pressure storage tank by utilizing a rod which penetrates the storage tank as a direct indicator for the position of the free piston will require the use of an additional and delicate dynamic seal. The tightness of the seal can never be guaranteed, not to mention the fact that undesirable frictional forces will arise between the indicating rod and the seal.
The Swiss Pat. No. 468,592 discloses a method to monitor the gas volume of a hydropneumatic storage tank which requires the use of a pump to fill the unit with hydraulic fluid. A manometer measures the storage tank pressure in order to determine a functional relationship between the pressure gradient which exists during the operational filling of the storage unit and the volume entering the storage unit. The method of the Swiss patent determines either the maximum permissible pressure difference, assigned to a specific filling volume, or the permissible minimum volume, assigned to a specific pressure difference. The monitoring of the filling volume is accomplished in the case of this method by utilizing the time period of the pump operation.
It is also known to count a specific number of revolutions by the pump shaft with a reduction gear. The gear can then actuate a switch when the pump shaft has performed a specific number of revolutions. If, due to a leak in the storage tank, a loss in gas volume occurs, the volume will drop below a certain magnitude. Then the pressure or respectively the pressure difference based on a specific filling volume will exceed a permissible maximum value and the loss of gas can be signaled by a pressure switch contact. The measurement of the pressure gradient based on a predetermined pump running time is subject to error, however, because the filling volume produced by the pump will vary since it depends on the existing temperatures. If oil is being used as the hydraulic fluid, the filling volume also depends on the viscosity of the oil. Fluctuations in the electric voltage of the pump motor will also lead to the delivery of dissimilar quantities within identical time units. Furthermore, wear and tear of the pump will influence the quantity being delivered. Finally, the hysteresis and the repetitive stability of pressure switches is such that they will not always switch at precisely the same trigger slack.
Accordingly, it is an object of the present invention to monitor, in the most simple manner, with a minimum number of maintenance-free devices, and continuously and free of error, the gas volumes of hydropneumatic storage units of an energy-storing system for the purpose of rapidly detecting and reporting a loss of gas volume.
The present invention accomplishes this and other objects by determining the state of flow of the hydraulic fluid entering the several hydropneumatic storage tanks while the pump is running. The coincidence of a running pump and a stoppage of flow is the criterion that is used by the monitoring system to detect a loss of compressible gas volume.
In a preferred embodiment of the invention, an appropriate pipeline supplies hydraulic fluid to the individual hydropneumatic storage tanks. The tanks are fed in parallel by way of a central pipeline with at least one flow monitor and an alarm which can be triggered by the flow monitor.
It will be especially advantageous if the movable partition in the storage tank is formed by a free piston, and if the housing or respectively the pressure cylinder of the storage tank serves as the guide for the free piston. In this way, the presence of a specifically selected gas volume reserve for the operational requirements of the energy-storing system or respectively the appropriate utilization of this system is maintained. For this same reason it will be further advantageous if the housing or respectively the pressure cylinder of the hydropneumatic storage tank which serves as the guide for the free piston, and holds a gas volume that is determined by the specific position of the free piston at any given time, will be in continuous communication with another gas storage tank through a connecting pipeline that is open at all times.
Finally, it is advantageous if the pump is equipped with a second signal device that is controlled by the running pump.
The present invention eliminates the disadvantages of the known systems and arrangements and in particular has the further advantage that it can be consummated by rather simple devices that are safe in operation and are commercially available. The installation, especially of the flow monitors, in already existing systems can be accomplished with a minimum of effort. A particular object of the present invention is that the monitoring system according to the present invention is especially distinguished by its independence from upsetting exterior as well as interior influences such as temperature, fluctuations in electric voltages, or in pump revolutions, irregularities due to wear and tear of the pump, the consequences of pressure switch hysteresis or the respective stability of such switches with respect to their triggering.